Fluid stream deflecting members for aircraft bodies or the like

ABSTRACT

Devices to reduce the drag experienced by aircraft and other travelling bodies, comprising sail-like members mounted on the body surface. These members project into the local stream that forms close to the surface during motion, and tend to divert that local stream back into the free stream direction and in so doing to experience useful thrust. The members are cambered and the camber varies from root to tip to allow for change in the local stream direction as distance from the surface increases. Special sails for use on surfaces subject to some complex flows, adjustable sails and arrays of several sails for use especially at aircraft wing tips, are also proposed.

This invention relates to devices to reduce the drag experienced bycraft in motion relative to masses of fluid, and especially by aircraft,and in particular to reduce the drag resulting from the fact that inproducing lift, various parts of an aircraft create local streams orflows whose directions are different from that of the free stream. Thisis particularly the case near to the wing tips of conventional aircraft.By "free stream direction" we mean the direction of the air relative tothe aircraft, measured well ahead of the aircraft. It is well known tomount flat fins at or close to such tips to obstruct the prominent localflow of air which in flight passes around such tips from the lower sideof the wing to the upper, but the usefulness of such fins has generallybeen confined to destroying this drag-inducing local flow; the fins haveseldom been shaped so as to generate any more positively useful forcesby reason of their reaction with the flow. In a few cases it has beenproposed to shape wing fins so that their reaction with local flowsgives rise to such positive forces, but in such cases it appears thatthe barrier function of the fins was still uppermost in the minds of thedesigners and that the possibility of achieving much more substantialbenefits by the correct shaping and setting of the fins was notappreciated.

The present invention results from considering such possibilitiesfurther, and in particular from considering the possibility of fins soshaped that they might generate thrust in a manner analogous to that ofa close-hauled sail on a boat. The invention will now be described, byway of example, with reference to the accompanying drawings in which:

FIG. 1 shows a device in elevation;

FIG. 2 comprises sections on the lines A and B in FIG. 1;

FIG. 3 is a plan view of an aircraft wing tip and tip tank fitted withsuch a device;

FIG. 4 is a front elevation of the parts shown in FIG. 3;

FIGS. 5 and 6 are graphs;

FIG. 7 shows a variable sweep device diagrammatically;

FIG. 8 is a diagrammatic plan view of devices fitted to a delta wing:

FIG. 9 is a front elevation of the parts shown in FIG. 8;

FIG. 10 is a diagrammatic perspective view of an aircraft wing tipfitted with four devices;

FIGS. 11 and 12 are diagrammatic plan views of alternative devices, and

FIG. 13 is a diagrammatic front elevation of a further alternativedevice.

The device shown in FIG. 1 is a rigid member 1 (which will be referredto as a sail) having a root end 2 and tip end 3 and carrying at its rootend a stub 4 by which it may be mounted upon an aircraft surface. Thesail is of aerofoil section from root to tip. The tangents to the sailcentre (or camber) line 7 at the trailing edge 5 lie in a common planethroughout the length of the sail, and the two transverse sections ofFIG. 2 in planes A and B show that the tangent 6 to line 7 at leadingedge 8 in plane A makes a relatively great angle (about 20° in theexample drawn) to the plane of the tangents to line 7 at the trailingedge 5, whereas at plane B close to tip 3 the corresponding angle ismuch less, only about 3° in the same example. Between root and tip thisangle falls progressively.

FIGS. 3 and 4 show three such sails mounted on a tip tank 10, itself inturn mounted upon the tip of a conventional-type wing 11 of an aircraft.Track 12 illustrates the path of a local stream of air that is createdclose to the surfaces of wing and tank during flight. This path leadsfrom the underside of the wing with rearward and rotary components toits motion. It passes up and around the wall of tank 10 and then headsinwards and backwards toward wing 11, but is intercepted by the leadingsail which turns it into the direction of the tangent 13 to the sailcentre line at the trailing edge of the sail, which is so mounted thatin a typical condition of flying, e.g. normal cruising altitude, thetangents along the whole trailing edge of the sail, from root to tip,lie parallel to the free stream. In practice the set of the trailingedge tangents will be chosen so as to offer the best overallcontribution to the flight performance of the aircraft, having regard tothe need to offer a substantial benefit in some flying conditions whileavoiding creating a serious liability in any others. In turning thestream the leading sail experiences a force in direction 14 after themanner of a close-hauled boat sail, and this force may be resolved intoa forward component which helps to propel the aircraft. The two othersails shown in FIGS. 3 and 4 work similarly upon the similar regions ofthe local stream that they intercept, and the sails are staggered asshown not only to avoid the forward sails spoiling the impact of thelocal stream upon the more rearward--for instance by the rearward sailslying within the wake of the forward--but also possibly to createpositively favourable interference effects. Tests with such a three-sailarrangement have been conducted with the rear sail located at about 75%of the wing tip chord and projecting horizontally as shown, the forwardsail at about 40% wing tip chord and pointing upwards and the third sailmidway between the others in both wing tip chord position andinclination. FIGS. 3 and 4 illustrate that while the part of the localstream close to the wing and tank turns sharply around tank 10, and thuslies at a comparatively great angle to the free stream direction andneeds a strong camber at the root of the sail to turn it, the weakerlocal stream 12a, which is further from the surfaces of the wing andtank, is less angled to the free stream direction and this only requiresthe lesser camber of the sail tip to straighten it. FIG. 5 shows theuncorrected drag coefficients for three tested aircraft wing designs,plotted graphically against lift coefficient. Each tested wing carried atip tank as in FIGS. 1 and 2, but one of the designs carried three sailsarranged as in FIGS. 3 and 4, another carried only one with its spanhorizontal and located at about 75% wing tip chord, and the third designcarried no sail at all. Since the drag coefficients are uncorrected forrig tare drag the graphical zero must be considered a false one. At zerolift coefficient the sails increased the drag coefficient dramaticallyby eight drag counts per sail. This is not surprising since the sailswere cambered and mounted in a manner appropriate to a lift coefficientof about 0.7. Flow visualisation tests at zero wing incidence showedthat a severe separation occurred over most of the inner concave surfaceof the sails. However this disappeared quite rapidly with increase inwing incidence and at an overall lift coefficient of 0.8 the three sailsper tank had reduced the drag coefficient by 108 counts whilst thesingle sail per tank had reduced the drag coefficient by 71 counts.

The effect of the sails can be seen more clearly in FIG. 6 where thedifference in drag coefficient between a lift condition and the zerolift condition is plotted against the square of the lift coefficient.This shows that the sails reduced the lift-dependent drag dramatically,the single sail increasing the effective aspect ratio of the wing by 23%whilst the three sails increased it by 46%.

FIGS. 1 to 6 all illustrate models of devices and aircraft wings, andtests upon them. FIGS. 1 and 2 are approximately to full scale, and theillustrated sail had a span-to-mean-chord ratio of 3.5; the nominalthickness chord ratio was 15%, and the taper ratio 0.4. FIGS. 3 and 4are to a scale of 0.4:1 approximately, and the area of each sail was0.6% of that of the aircraft wing. FIGS. 5 and 6 relate to tests of thismodel in an 8'×6' wind tunnel at a Reynolds number of 1.1×10⁶ based onwing mean chord or about 1.1×10⁵ based on sail mean chord.

FIG. 10 shows another application to an aircraft wing tip, viewed from aposition below the wing, and both forward and outboard of it. Because ofthe possible structural difficulty of anchoring the four sails 41 to 44securely to an ordinary wing tip, the original tip of wing 45 has beencut away to receive a robust body 46 to which the roots 47 of the sailsare anchored; a body such as 46 may of course be omitted if secureanchoring is possible without it. The profile of body 46 is such thatwhen the body is in place it gives to the wing a tip profile as nearlysimilar as possible to the original, the small cylindrical tail 48 ofthe body having little aerodynamic effect other than that of fairingwhat would otherwise be a bluff body. Sails 41 to 44 are so mounted onbody 46 that their edges 49 all pass through the fore-and-aft axis 50 ofthe body, which is effectively the chord-line of the wing tip. The lineof each trailing edge 49 passes through the axis 50 and in tests for thereduction of the lift-dependent drag of the wing the best results wereobtained when the difference in the angular setting of adjacent sailsabout axis 50 was in the region of 10°-20°. For the reduction oflift-dependent drag, best results were also obtained when the wholearray of sails lay as nearly as possible horizontal, that is to say thetrailing edges of sails 42 and 43 lay on opposite sides of the plane ofthe wing, and equally inclined to it. Sails with spans of up to about50% of wing tip chord were tested, and results of the best overallpromise were obtained when the span of each sail was about 25%especially 24% of the wing tip chord C, and when the root chord c ofeach sail was about 16% of wing tip chord C, so that the whole array ofsails had a total chord of rather over C/2. Tests have suggested thatwith greater or lesser numbers of sails the ratio of the total of theroot chords c of the individual sails to the wing tip chord C shouldstill generally lie between about one-half and three-quarters, althoughpossibly rising to full wing tip chord in certain cases.

A fixed geometry sail will have a camber distribution which is optimumfor only one aircraft incidence. At other incidences the turning of thelocal flow by the sail is partly by incidence effects rather than bycamber. This tends to produce a pressure distribution with a muchsharper "peak", and so with the danger of flow separation, particularlyat high sub-sonic flight speeds. A camber distribution which varied withaircraft incidence might be an ideal solution, but this might be bothcostly and heavy in some instances.

A possible alternative solution is to mount the sails so that they canbe turned, relative to the aircraft surface, about their span axes. Thissolution is illustrated in FIG. 11: the span axis of sail 20 projectssubstantially horizontally outwards from the tip 21 of an aircraft wing22. The sail is mounted on a shaft 23, which is rotatable by a motor 24controlled by a device 25 responsive to aircraft incidence. The reasonswhy such turning may enable the sails to act more effectively atincidences other than the original one may be summarised by stating thatthe ideal variation in camber, from root to tip, changes with aircraftincidence. Varying the root setting of a sail with a fixed spanwisecamber distribution allows a compromise to be achieved between highthrust on the sail and low sail complexity.

Another alternative could be to mount the sails so that they are capableof being retracted into, or projected further from the aircraft surface.Such projection or retraction may help the sails to match changes ofincidence more effectively than fixed sails can, because for wingtip-mounted sails the variation in chamber from root to tip decreaseswith decrease in aircraft incidence as does the distance from the rootover which a useful thrust force can be generated. By withdrawing partof the sail into the wing tip as the aircraft incidence is reduced, orextending the sail from the wing tip as incidence increases, anapproximate matching of sail camber to local flow direction can beachieved. This solution is illustrated in FIG. 12 where the sail 30 ismounted on a rack 31 engaging with a pinion driven by motor 32, which iscontrolled by a device 33 respective to aircraft incidence. By drivingthe rack 31, the motor 32 can move sail 30 between its fully extendedposition (as shown) and a fully retracted position in which it liesentirely within a recess 34 formed within the tip of wing 35.

Another possibility, for multi-sail arrangements mounted on a wing tipbody and thus of the general kind shown in FIGS. 3 and 4, is to mountthe body for rotation about a fore-and-aft axis. Such an arrangement isshown in FIG. 13 where the body 60 on the tip of an aircraft wing 67carries three sails 61, 62 and 63 and is rotatable about fore-and-aftaxis 64 by a motor 65 controlled by a device 66 responsive to aircraftincidence. The sails are shown in full lines in the setting they mayoccupy for normal flight. For high lift, however, motor 65 may rotatebody 60 through 75°, say, so that the sails occupy their dotted linepositions.

A further alternative solution is suggested by noting that the angle ofthe local flow direction to that of the free stream, expressed as afraction of the wing incidence, varies regularly with distance from thetank. Thus if the sails have variable sweep, the sweep angle may bechanged with aircraft incidence in such a way that the active span ofthe sail, that is to say the span actually projecting from the wing,presents substantially the correct camber (for the chosen aircraftincidence) from one end to the other. As the speed of the aircraftincreases, its incidence will decrease and the sails can be swept more.At extreme sail sweep angles the camber would tend towards an anhedraleffect rather than a camber and thus would not be likely to induce highlocal velocities. FIG. 7 suggests a possible sweep variation; in thisFigure the sail 71 is rotated about axis 72 by motor 73 and to keep thesweep variation small, the most forward position corresponds to about30° sweep and the most aft to about 70°.

The forward thrust on such sails is likely to be a maximum when aircraftspeed is low and incidence is high since the sails should provide athrust proportional to the lift-dependent drag of the aircraft. A simplespring--`g` feel control system for the sweep angle may be satisfactory.

The sail principle can be applied to any condition when the local flowis at an angle to the free stream direction. One obvious group ofexamples of this is when vortex flow is generated by a low aspect ratiobody, a slender wing or strake at incidence. Consider the case of aslender wing with moderately sharp leading edges. Then at all but smallincidences a pair of spiral vortex sheets will spring from the leadingedges, causing severe changes in the local flow directions inboard ofthe leading edge. If towards the trailing edge, or just behind it,vertical sails 74 are erected, as shown in FIGS. 8 and 9, through thecore of the vortex, they will experience a marked sidewash, varying fromoutwards near the wing surface to inwards above the core of the flowsand in tending to unwind the vortex would experience a thrust whichcould be a significant fraction of the drag associated with that vortex.Although the scale of the Figures does not permit this to be shownclearly, typical sails of the kind shown in FIGS. 8 and 9 may have acamber of one sense at the root, and on progressing from the root to thetip that camber first increases to a maximum, then falls to zero in theregion of the predicted core of the vortex, then rises in the oppositesense and finally falls to near zero at the tip.

If the sail was pivoted at the zero camber section associated with thecentre of the vortex sheet then a variation of sweep about this pointwould go a long way towards matching the camber distribution to thevariation is sidewash associated with changes in wing incidence. Thelateral shift of the vortex sheet centre with change of incidence islikely to be only a secondary effect. Again, the high speed, low wingincidence case corresponds to the most swept position of the sails.

Although the invention has been described with reference mainly to sailswith their spans substantially at right angles to the surfaces on whichthey are mounted, it applies equally to wings of sweptconfiguration--both fixed sails and also sails that are swept inattitude but movable like, for instance, the sails shown in FIGS. 11 and12. The invention applies also to sails mounted on a variety of surfacesof craft in motion within and relative to masses of fluid. For instancenot only whole aircraft wings, but also parts of whole-span flaps whichare deflected to give an aircraft extra lift at take-off and landing: insuch a case the sails could be attached to such flaps at their outboardends, or to the stationary wing near these ends. It is contemplated thatit might be necessary to arrange that such sails became operative onlywhen the flaps themselves were operative, and were retracted orotherwise made inoperative at other times to avoid then interfering withthe proper flow of air over the wing. The invention applies also tosurfaces of stationary craft dependent on gaseous movement (e.g. thevanes of windmills), to the surfaces of boats, water wheels and otherapparatus dependent on the movements of non-gaseous fluids, to rotaryand other moving surfaces of aircraft such as the blades of helicoptorrotors, and to aerofoil surfaces on ground vehicles, e.g. racingvehicles. For helicopters the invention may have particular uses inreducing the effect of the vortices which form behind each blade, andwhich in the descent mode may cause adverse blade-vortex interference.Another consequence of the "vortex unwinding" behaviour of the sails isthat it not only reduces the lift-dependent drag, but also reduces theinitial strength of the trailing vortex and causes it to diffuse morerapidly as downstream distance increases. This suggests that use of thepresent invention may allow aircraft to fly more closely behind othersthan is now possible, thus reducing separation times at airfields. Areduction in tip vortex strength is also likely to be of benefit tocrop-spraying aircraft, operating close to the ground.

I claim:
 1. A body intended for motion within and relative to a mass offluid, in which:said body presents a surface which will in use form aninterface with said mass of fluid; an array comprising a plurality ofmembers of cambered aerofoil cross-section having mountings upon saidsurface so as to project outwards from said surface into the space wherea local stream of fluid will form in use, whereby in such use saidmembers will tend to divert said local stream back into the free streamand in so doing experience useful thrust, and in which the said camberof each said member varies to allow for the change in the direction ofthe said local stream as distance from said surface increases; and saidmembers are staggered so as to have more forward, middle and morerearward members in the direction of the relative motion of said bodyand said fluid so that the more rearward of said members avoid the wakeof the more forward of said members, and so that the impact of the moreforward members upon the local stream causes a more favorable incidenceof the local stream upon more rearward members than would exist if saidmore forward members were absent; said body being an aircraft winghaving a tip with said members projecting outwards from said tip; saidmountings of said members being staggered in a fore-and-aft direction,and in which each said member has a span of less than about 50% of thechord of said wing tip, and in which the total root chords of saidmembers equal between about one-half and three-quarters of said wing tipchord.
 2. A body intended for motion within and relating to a mass offluid according to claim 1 in which said root chord of each said memberis between about 10% and 20% of said wing tip chord.
 3. A body intendedfor motion within and relating to a mass of fluid according to claim 2in which said root chord of each said member is about 16% of said wingtip chord.
 4. A body intended for motion within and relating to a massof fluid according to claim 1 in which said mountings are also staggeredto lie along at a portion of a spiral path which is opposite to that ofthe normal local flow around said tip in use.
 5. A body intended formotion within and relating to a mass of fluid according to claim 4 inwhich the difference in angular setting between adjacent said members insaid spiral stagger is between about 10° and about 20°.
 6. A bodyintended for motion within and relating to a mass of fluid according toclaim 5 in which said difference is about 15°.
 7. A body intended formotion within and relating to a mass of fluid according to claim 5 inwhich the middle members of said array project outwardly from said wingtip in a direction similar to that of the span of said wing itself.
 8. Abody intended for motion within and relating to a mass of fluidaccording to claim 5 in which the middle members of said array point ina direction substantially at right angles to that of the span of saidwing itself.
 9. A body intended for motion within and relating to a massof fluid according to claim 5 in which said array as a whole is mountedrotatably about a fore-and-aft axis, whereby said array may be rotatedto different settings to suit different flight conditions of saidaircraft.
 10. A device to reduce the drag experienced by bodies inmotion within and relative to masses of fluid, comprising:a bodyintended for motion within and relative to a mass of fluid andpresenting a body surface; a member of cambered aerofoil cross-sectionhaving a given length mounted upon said surface so that the span of saidmember projects outward from said surface into the space where a localstream of fluid will form in use, whereby in such use said member willtend to divert said local stream back into the free stream and in sodoing experience useful thrust; in which said camber of saidcross-section of said member varies to allow for change in the directionof said local stream, the variation in said camber occurring over thelength of said member from a high degree of camber in one sense nearsaid surface, said degree of camber first falling to zero along saidlength and then increasing in the opposite sense near the tip end ofsaid span of said member.
 11. A device to reduce drag as claimed inclaim 10 wherein said camber of said member falls to near zero in thevicinity of the tip of said span of said member.